The present invention relates to a process for depositing a semiconductor film of amorphous hydrogenated silicon or an alloy of amorphous hydrogenated silicon on a substrate in a plasma chamber containing at least one pair of electrodes connected to an electric high-frequency generator, wherein the substrate is connected to one electrode, the substrate being placed at a distance d from the other electrode, a gas containing at least one silicon compound is introduced into the chamber, and high-frequency electrical power is applied to the electrodes to produce a plasma between the electrodes. The invention also relates to an installation for implementing this process.
The deposition of amorphous hydrogenated silicon (a-Si:H) in a plasma reactor is described especially in U.S. Pat. No. 4,226,898 and it is generally employed, starting for example from compounds of the silane type (SiH.sub.4,Si.sub.2 H.sub.6, etc.) and doping gases, to make photodetector elements or photovoltaic cells. Amorphous layers of hydrogenated silicon alloys are also deposited, for example according to the formulae a-Si.sub.x Ge.sub.1-x :H, a-Si.sub.x C.sub.1-x :H or a-Si.sub.x N.sub.1-x :H. In industrial applications one primarily aims to obtain adequate product quality, in particular a limited number of defects per unit volume, which defects produce localized states in the energy gap of the semiconductor, with as high a deposition rate as possible, hence at moderate cost. The plasma is produced in most cases by radiofrequency (RF) discharges between two electrodes, one of which carries the substrate, the plasma thus being capacitively coupled with a high-frequency generator.
The parameters affecting the deposition process are numerous and encompass notably: the interelectrode gap, the substrate temperature, the gas mixture employed, the pressure and the rate of gas introduction into the chamber, the power and the frequency of the RF discharge. It is moreover important to confine the plasma as far as possible to the zone situated between the two electrodes, which may be achieved especially by means of an appropriate choice of the following parameters: pressure, interelectrode distance, RF power.
To obtain a high deposition rate, one should principally increase the concentration of the plasma and/or the RF power density per unit surface of the substrate. However, the increase of these parameters has hitherto encountered limits due to the increase of the density of defects in the deposited film, that is, the number of defects per unit volume, in particular on account of a phenomenon of gas phase polymerization, so that the best deposition rates presently achieved at the industrial stage do not generally exceed 0.4 nm/s (4.ANG./s) for a defect density N.sub.s, measured by the PDS (Photothermal Deflection Spectroscopy) method, in the order of 1-5.times.10.sup.16 /cm.sup.3 for a film having a thickness in the order of 1 .mu.m. Thus, the deposition of such a film lasts at least three quarters of an hour. Said PDS method is described by W.B. Jackson and N.M. Amer in Phys. Rev. B, 25, p. 5559-5562.
An example of attempts to increase the deposition rate is given by T. Hamasaki et al in Appl. Phys. Letters 44(6) 1984, p. 600-602. In this case, confinement of the plasma by a grid surrounding the zone of the electrodes and earthed enabled to obtain deposition rates of the order of 4 to 5 nm/s (40 to 45 .ANG./s while utilizing pure silane gas (SiH.sub.4) and a standard frequency of 9 MHz. Nevertheless, except for the conductivities in dark and under illumination, the authors do not give any precise indications regarding the quality of the deposit obtained and one may expect a relatively high value of N.sub.s.
U.S. Pat. No. 4,406,765 describes a process wherein the plasma is produced by superposition of a DC electric field and an electric field of high-frequency between 0.1 MHz and 100 MHz (or a pulsed electric field), in order to obtain above all a very high deposition rate. The quality of the film obtained is not indicated in detail: it should be expected to be inferior to that which is generally required for solar cells. This publication also does not indicate if certain frequency values are preferable in the very large range proposed. It may thus be presumed that a modification of the frequency has no appreciable effect in this technique.
The object of the present invention is to provide simple and effective means enabling to improve known processes of the type indicated above, in such a manner as to increase the deposition rate without increasing the number of defects, or while even reducing it with respect to deposits obtained by conventional processes.